The U.S. Food and Drug Administration (FDA) has revealed preliminary findings from a multi-year environmental study of a specific growing region in the Southwest U.S., which sought to better understand the ecology of human pathogens in the environment. The research was conducted in partnership with the Arizona Department of Agriculture, and in conjunction with the University of Arizona Cooperative Extension, the Wellton-Mohawk Irrigation and Drainage District (WMIDD), and members of the leafy greens industry.

From May 2019—April 2024, environmental samples were collected from irrigation and other surface waters, soil, sediment, air dust, animal fecal material, wildlife scat, plant tissue, and other sources across an approximately 54-mile area within the growing region. Through repeated sample collection, testing, measurement, and analysis, the team was able to observe variability of pathogens in this region over time, providing new insights about pathogen prevalence, location, and potential influencing factors, such as seasonality, weather conditions, and animal activities on adjacent and nearby land.

The research team is continuing to analyze data from this study, but preliminary findings suggest:

• Shiga toxin-producing Escherichia coli (STEC) strains detected in water, sediment, and plant tissue genetically matched strains isolated in air samples, providing evidence that STEC in air can transfer to other locations and surfaces
• Air samples collected during the study yielded positive viable pathogen results indicating that bacteria can survive in the air and that dust can act as a transfer mechanism for both pathogens and indicator organisms from adjacent and nearby land to water, soil, and plant tissue
• Surface water evaluated in the study experienced a change in water quality and an increase in the prevalence of STEC as water moved past a nearby Concentrated Animal Feeding Operation (CAFO), even in the absence of surface run-off or other direct contamination, which indicates that airborne disposition of dust from the CAFO was potentially a factor in the contamination of the irrigation water
• Distance played an important factor in the likelihood of STEC being detected in collected airborne dust, with the percentage of positive samples declining steadily as air sampling moved in an incremental manner away from concentrated animal operations
• Birds and other wildlife do not appear to be significant sources of STEC or E. coli O157:H7 in or around the part of the Southwest growing region evaluated; however, continued monitoring is warranted to reduce potential risk to produce, the environment, and water sources.

The study was initiated following the 2018 outbreak of E. coli O157:H7 linked to the consumption of leafy greens grown in the Southwest U.S. growing region. During the initial environmental assessment into that outbreak, FDA explained that the most likely way the lettuce had been contaminated was from the use of contaminated water from an irrigation canal; however, the agency noted that it could not rule out that there might be other sources or means of contamination that were not identified during the investigation. Importantly, while findings do suggest that airborne dispersal of the outbreak strain was likely a contributing factor, the study did not identify the specific source and route of contamination that contributed to the 2018 outbreak.

FDA shared that members of the Arizona leafy greens industry will be working through the Desert Food Safety Coalition to continue to explore the findings from the study and to improve food safety. This group intends to collaborate with the Arizona Department of Agriculture, University of Arizona Extension, Yuma Fresh Vegetable Association, Yuma Safe Produce Council, Arizona Farm Bureau, Arizona Leafy Green Marketing Agreement (AZ LGMA), Western Growers, USDA-APHIS Wildlife Services, Arizona Cattle Feeders’ Association, additional grower and landowner coalition members, and other agricultural community stakeholders. While the results are regionally specific, the findings may also help address some knowledge gaps identified in the Leafy Greens STEC Action Plan, particularly concerning animal activities on adjacent and nearby land.

The research team intends to present additional details about this study during the International Association for Food Protection (IAFP) Annual Meeting, taking place July 14–17, 2024 in Long Beach, California.